Genetically engineered toxoid molecules have been evaluated as potential protein carriers for pneumococcal capsular polysaccharide in a conjugate vaccine. Immunization of mice with genetically engineered toxoids protected mice from subsequent challenge with virulent pneumococci to a greater extent that wild type toxin. An inactivated pneumolysin, "pneumolysoid B (PdB)" was prepared from a mutant gene constructed by the site-directed mutagenesis of type 1 ply gene, pJCP20 plasmid in which DNA fragment that encodes for Try433 of pneumolysin was changed to Phe. Conjugation of the toxoid to pneumococcal type 19F PS enhanced the immunogenicity of both PS and pneumolysoid antigens. The 19F PS-PdB conjugate was given to maternal mice during gestation and/or lactation and an additional immunogen was given to young after birth. Two weeks after last injection, the immunized young mice were challenged with 19F pneumococci. Almost all young mice in control group not receiving immunogen, died within first 3 days after challenge (5% survival rate), while most mice in the immunized group survived (71.4-75%0. Thus, the injection of mice with 19F PS conjugated with inactivated pneumolysin has conferred protective immunity to pneumococcal infection.